J. Robert Oppenheimer is the embodiment of rejection of groundrules and existing environment. His brilliant thought and unwillingness to accept the status quo caused a prolific shift in the way the world consumes science, violence, and the morality of war. On one hand, his defiance of the existing modes of science made possible the creation of the atomic bomb, which did ultimately end World War II.
However, at what cost? Following the end of the war and Oppenheimer's battle with the American Government, he observes that sin is now forever tied to physics. And terms like "mutually assured destruction" are now commonplace in the language of war and violence. His work developing the bomb was shrouded in secrecy before, and afterwards he notes that knowledge should be shared, a concept which children know better than anyone else.
McLuhan chooses to include a quote from Oppenheimer because he represent a important moment in which the future of the world and humanity was, for the first time, in question. Because he broke boundaries and shifted the way that media-- and, subsequently, humankind-- consumed our own collective mortality. And to demostrate the ways in which children are the one existing force of morality and collective thought not entirely created by media-- although this does seem to be changing as technology and media is more and more accessible to younger and younger children.